1. Field of the Invention
The present application relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device and a method of fabricating the in-plane switching mode liquid crystal display device.
2. Discussion of the Related Art
As information age progresses, display devices processing and displaying a large amount of information has been developed. Specifically, flat panel display (FPD) devices have been required for satisfying characteristics such as light weight, thin profile, and low power consumption. As a result, a liquid crystal display (LCD) devices having advantages in color reproducibility and profile have been suggested.
The LCD device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Since the liquid crystal molecules have long thin shapes and a pretilt angle for alignment, the alignment direction of the liquid crystal molecules can be controlled by changing the pretilt angle due to an applied voltage. Accordingly, the alignment of the liquid crystal molecules changes in accordance with the alignment direction of the liquid crystal molecules controlled by applying a voltage to a liquid crystal layer. As a result, the image is displayed by modulating a polarized light due to the optical anisotropy of the liquid crystal molecules. Among several types of LCD devices, active matrix LCD (AM-LCD) devices where thin film transistors (TFTs) and pixel electrodes connected to the TFTs are disposed in matrix are currently widely used because of their high resolution and superior quality for displaying moving pictures.
A related art LCD device includes a color filter substrate having a common electrode, an array substrate having a pixel electrode, and a liquid crystal layer interposed between the color filter substrate and the array substrate. In the related art LCD device, the liquid crystal layer is driven by a vertical electric field between the pixel electrode and the common electrode. The related art LCD device provides a superior transmittance and a high aperture ratio. However, the related art LCD device has a narrow viewing angle because it is driven by the vertical electric field. To overcome the above disadvantages, various other types of LCD devices having wide viewing angles, such as in-plane switching mode (IPS) mode LCD device, have been developed.
FIG. 1A is a cross-sectional view of the related art in-plane switching mode liquid crystal display device in an ON state, and FIG. 1B is a cross-sectional view of the related art in-plane switching mode liquid crystal display device in an OFF state.
In FIGS. 1A and 1B, an in-plane switching (IPS) mode liquid crystal display (LCD) device 10 includes a first substrate 20 having a thin film transistor (not shown), a second substrate 30 having a color filter layer and a black matrix and a liquid crystal layer 40 between the first and second substrates 20 and 30. A common electrode 22 and a pixel electrode 24 are alternately formed on the first substrate 20. An electric field E is generated according to a voltage applied to the common electrode 22 and the pixel electrode 24, and liquid crystal molecules 40a and 40b of the liquid crystal layer 40 are re-aligned along the electric field E.
In the ON state of FIG. 1A, the voltage is applied to the common electrode 22 and the pixel electrode 24, and the electric field E is generated. The electric field E has a vertical portion directly over the common electrode 22 and the pixel electrode 24, and a horizontal portion between the common electrode 22 and the pixel electrode 24. Accordingly, the first liquid crystal molecules 40a directly over the common electrode 22 and the pixel electrode 24 are not re-aligned, and the second liquid crystal molecules 40b between the common electrode 22 and the pixel electrode 24 are re-aligned along the electric field E. Since the liquid crystal layer 40 between the common electrode 22 and the pixel electrode 24 is re-aligned along the horizontal portion of the electric field E in the ON state, the IPS mode LCD device 10 displays images with a wide viewing angle. For example, the images may be displayed with a viewing angle of about 80° to about 85° along top, bottom, right and left directions with respect to a normal direction of the IPS mode LCD device.
In the OFF state of FIG. 1B, since the voltage is not applied to the common electrode 22 and the pixel electrode 24, and the electric field E is not generated. Accordingly, the first and second liquid crystal molecules 40a and 40b of the liquid crystal layer 40 are not re-aligned.
In the IPS mode LCD device according to the related art, a polarization state of the light passing through the IPS mode LCD device is adjusted by first and second polarizing plates on outer surfaces of the first and second substrates, respectively.
FIG. 2 is a cross-sectional view showing an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 2, an in-plane switching (IPS) mode liquid crystal display (LCD) device 10 includes first and second substrates 20 and 30 facing and spaced apart from each other, and a liquid crystal layer 40 between the first and second substrates 20 and 30. Although not shown in FIG. 2, a common electrode and a pixel electrode generating an electric field are formed on an inner surface of the first substrate 20.
In addition, first and second polarizing plates 52 and 54 are formed on outer surfaces of the first and second substrates 20 and 30, respectively. The first polarizing plate 52 includes a first supporting layer 52a, a first polarizing layer 52b and a first protecting layer 52c, and the second polarizing plate 54 includes a second supporting layer 54a, a second polarizing layer 54b and a second protecting layer 54c. Each of the first and second supporting layers 52a and 54a includes tri-acetyl cellulose (TAC) having a retardation of about 0. Each of the first and second polarizing layers 52b and 54b determining a polarization property is formed by stretching poly-vinyl alcohol (PVA) adsorbing iodine (I) or dye. Each of the first and second protecting layers 52c and 54c also includes TAC.
When the first and second polarizing plates 52 and 54 are disposed such that absorption axes of the first and second polarizing plates 52 and 54 are perpendicular to each other, the IPS mode LCD device is operated in a normally black mode. Accordingly, when the IPS mode LCD device 10 has an OFF state, the horizontal electric field is not generated between the common electrode and the pixel electrode, and the liquid crystal molecules of the liquid crystal layer 40 are not re-aligned so that the incident light can penetrate the liquid crystal layer 40 without polarization. As a result, the linearly-polarized light passing through the first polarizing plate 52 to have a polarization axis perpendicular to the absorption axis of the first polarizing plate 52 penetrates the liquid crystal layer 40 without change of polarization state, and is completely absorbed by the second polarizing plate 54 having an absorption axis perpendicular to the absorption axis of the first polarizing plate 52, thereby a black image displayed.
When the IPS mode LCD device has an OFF state, although the black image is displayed along a normal direction of the IPS mode LCD device, the brightness of the black image along up, down, right and left oblique directions of the IPS mode LCD device may increase due to light leakage. The light leakage is generated because the absorption axes of the polarizing plates are not perpendicular to each other.
FIGS. 3A and 3B are views showing absorption axes of polarizing plates of an in-plane switching mode liquid crystal display device according to the related art when viewed at a normal viewing angle and at an oblique viewing angle, respectively.
In FIG. 3A, when an in-plane switching (IPS) mode liquid crystal display (LCD) device 10 (of FIG. 2) is viewed at a normal viewing angle, a first absorption axis ABS1 of a first polarizing plate 52 on an outer surface of a first substrate 20 (of FIG. 2) and a second absorption axis ABS2 of a second polarizing plate 54 on an outer surface of a second substrate 30 (of FIG. 2) cross each other with a first angle a1.
In FIG. 3B, however, when the IPS mode LCD device 10 is viewed at an oblique viewing angle, the first absorption axis ABS1 of the first polarizing plate 52 and the second absorption axis ABS2 of the second polarizing plate 54 cross each other with a second angle a2 greater than the first angle a1. Accordingly, the first absorption axis ABS1 of FIG. 3B for the obliquely incident light to the IPS mode LCD device 10 is counterclockwise rotated with respect to the first absorption axis ABS1 of FIG. 3A for the normally incident light to the IPS mode LCD device 10, and the second absorption axis ABS2 of FIG. 3B for the obliquely incident light to the IPS mode LCD device 10 is clockwise rotated with respect to the second absorption axis ABS2 of FIG. 3A for the normally incident light to the IPS mode LCD device 10.
As a result, the obliquely incident light to the IPS mode LCD device 10 passes through the first polarizing plate 52 and is linearly polarized to have a polarization state having a polarization axis PL perpendicular to the first absorption axis ABS1 of FIG. 3B. However, since the polarization axis PL is not parallel to the second absorption axis ABS2, the obliquely incident light is not absorbed by the second polarizing plate 54 to cause the light leakage. Therefore, when the IPS mode LCD device 10 is viewed along up, down, right and left oblique directions, the luminosity of the black image is deteriorated due to the light leakage, thereby contrast ratio reduced.
FIG. 4 is a Poincare sphere showing polarization states of obliquely incident light to an in-plane switching mode liquid crystal display device according to the related art, and FIG. 5 is a view showing a brightness contour line of a black image with respect to a viewing angle in an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 4, the obliquely incident light to an in-plane switching (IPS) mode liquid crystal display (LCD) device passes through a first polarizing plate 52 (of FIG. 2) and is linearly polarized to have a polarization state having a polarization axis PL perpendicular to a first absorption axis ABS1. Since a second absorption axis ABS2 of a second polarizing plate 54 (of FIG. 2) is not perpendicular to the first absorption axis ABS1, the polarization axis PL is located at different position in the Poincare sphere from the second absorption axis ABS2. Accordingly, the obliquely incident light does not display a complete black image and light leakage occurs.
In FIG. 5, the first polarizing plate 52 and the second polarizing plate 54 are disposed such that the first absorption axis ABS1 and the second absorption axis ABS2 are parallel to a horizontal direction and a vertical direction, respectively. When the IPS mode LCD device displays a black image, a complete black image without light leakage is viewed at a normal viewing angle having a polar angle θ of about 0° with respect to a z-axis normal to the IPS mode LCD device. However, a light leakage occurs at an oblique viewing angle along diagonal directions having azimuthal angles φ of about 45°, 135°, 225° and 315° with respect to an or a y-axis parallel to the IPS mode LCD device. Accordingly, brightness of the black image increases and contrast ratio is reduced at the oblique viewing angle of the IPS mode LCD device. For example, the black image may have relatively high brightness of about 0.018331 (arbitrary unit: A.U.) at an oblique viewing angle having a polar angle of about 60° and an azimuthal angle of about 45°. Although a compensation film having a complex retardation is used for the IPS mode LCD device to solve the above problems, the retardation film causes complication in fabrication process and increase in fabrication cost.